Method for production of oxide and silicon layers on a metal surface

ABSTRACT

The invention relates to a method of producing oxide and silicate layers on metal surfaces in a liquid electrolyte, particularly for aluminum metals, magnesium metals and their alloys, as well as for tantalum, titanium, niobium and zirconium. During the production of the oxide and silicate layers in the liquid electrolyte, a bipolar power source is used whose polarity can be changed.

FIELD OF THE INVENTION

The invention relates to a method of producing oxide and silicate layers on metal surfaces in a liquid electrolyte, particularly for aluminum metals, magnesium metals and their alloys, as well as for tantalum, titanium, niobium and zirconium.

BACKGROUND OF THE INVENTION

German Patent Document DE 41 04 847 C2 shows a method of ceramizing metal surfaces, by which metal parts are ceramized in a liquid electrolyte by spark discharge. In this case, the parts are connected to the multiphase periodic power source such that the parts alternately take over the function of the anode and cathode. A counterelectrode, which is not to be ceramized, is therefore not required.

In German Patent DD 299 074 A5, a lubricant on an inorganic base for pressing and drawing is disclosed which consists of a multilayer construction of characteristic oxides. For producing these oxide layers, the metal parts in a liquid electrolyte are subjected to a pulse voltage which is always rectified.

Furthermore, a surface protection for magnesium materials is known from the brochure “AHC Oberflächentechnik: Magoxyd-Coat” (“AHC Surface Engineering: Magoxide Coat”), in which a protective ceramic layer of magnesium oxide is applied to the surface of a magnesium part to be protected. The production of this layer takes place by anodic oxidation in a cooled, slightly alkaline electrolyte.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows a cross section showing layers on a metal.

SUMMARY OF THE INVENTION

An object of certain embodiments the present invention is to provide a method of producing oxide and silicate layers on metal surfaces, particularly for magnesium, aluminum and their alloys as well as for tantalum, titanium, niobium and zirconium, by means of which a protective layer against corrosion and wear can be produced economically.

This object is achieved in certain embodiments in that, during the production of the oxide and silicate layers in the liquid electrolyte, a bipolar power source is used whose polarity can be changed.

It was found that the layer buildup rate in the case of this method is higher than in the case of the known methods using direct current or pulsed current. Oxide and silicate layers of a thickness of more than 20 μm can therefore be produced more rapidly and cost-effectively.

Thus, the effect of the current takes place in the second or millisecond range. In-between, the pole reversal takes place in the millisecond range. As a result the density of the oxide and silicate layers can be noticeably increased. In addition, it is suggested that the pulse ratio be selected to be greater than 1:1; that is, the time period during which the part to be coated- is connected as a cathode is to be selected longer than the time period during which the part to be coated is connected as an anode.

Different layer sequences are permitted by means of successive coatings in different electrolytes.

An exemplary embodiment of the invention is described in detail in the following.

A forged motor vehicle wheel consisting of the magnesium alloy AZ80 is first cleaned in a bath of 10% ethanoic acid.

In a second step, the motor vehicle wheel is immersed in an electrolyte I consisting of an aqueous solution of potassium hydroxide (KOH) and sodium fluoride (NaF) and is connected with a power source. The power source supplies a current of between 30 and 100 A.

First, the motor vehicle wheel is connected as a cathode and is activated for approximately 40 seconds. Subsequently, the current is pulsed for 30 minutes such that, for a period of 30 ms, the motor vehicle wheel is connected as an anode and, for a period of 130 ms, is connected as a cathode. A layer of magnesium oxides and aluminum oxides is created which has a thickness of from 3 to 10 μm.

Then the further unipolar coating takes place in the electrolyte II consisting of an aqueous solution of KOH, NaF and sodium metasilicate (Na₂O₃Si). The wheel, immersed in the electrolyte II, is in each case connected for 55 ms with +250 V as an anode and for 500 ms at −250 V as a cathode. The entire treatment takes 20 minutes. During the treatment time, a layer of magnesium silicate is created which has a thickness of 10 to 20 μm.

Both electrolytes are tempered or cooled to 30 to 40° C.

The total layer of magnesium oxides and aluminum oxides and subsequently magnesium silicate produced in this manner and having a thickness of approximately 25 μm is tight with respect to corrosive media. The single figure is a micrograph of the total layer. According to the above-described method, an oxide layer 2 of a thickness of 3.25 μm consisting of magnesium oxides and a silicate layer 3 of a thickness of 11.61 μm consisting of magnesium silicate are applied to a wheel 1 consisting of magnesium.

As other alternatives, the following layer sequences can be produced:

Alternative A: By the successive use of electrolyte I, then electrolyte II and then again electrolyte I, an oxide layer, silicate layer, oxide layer sequence.

Alternative B: By the successive use of electrolyte II, then electrolyte I and then again electrolyte II, a silicate layer, oxide layer, silicate layer sequence.

The foregoing description and examples have been set forth merely to illustrate the invention and are not intended to be limiting. Since modifications of the described embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed broadly to include all variations within the scope of the appended claims and equivalents thereof. 

1. A method of producing oxide-silicate layers on a metal surface in a liquid electrolyte, said method comprising: connecting a bipolar power source to the metal surface, applying power and alternating the polarity of the power.
 2. The method of claim 1, wherein said metal surface comprises at least one metal selected from the group consisting of aluminum, alloys of aluminum, magnesium, alloys of magnesium, tantalum, titanium, niobium and zirconium.
 3. The method of claim 1, wherein the step of alternating the polarity of the power source takes place in the millisecond range.
 4. The method of claim 3, wherein the time period during which the part to be coated is connected as an anode is shorter than the time period during which the part to be coated is connected as a cathode or is without current.
 5. The method of claim 1, further comprising the step of controlling the layer sequences created.
 6. The method of claim 5, wherein the layer sequences created are controlled by selecting an electrolyte.
 7. The method of claim 1, wherein the metal surface is immersed in a first electrolyte during a time in which power is applied.
 8. The method of claim 7, wherein the first electrolyte comprises an aqueous solution of potassium hydroxide and sodium fluoride.
 9. The method of claim 7, further comprising the steps of immersing the metal surface in a second electrolyte during a time in which power is applied.
 10. The method of claim 9, wherein the second electrolyte comprises an aqueous solution of potassium hydroxide, sodium fluoride and sodium metasilicate.
 11. The method of claim 1, wherein power having one polarity is applied for a period of at least 30 milliseconds.
 12. A metal surface having oxide-silicate layers thereon, wherein said metal surface is produced by the process of claim
 1. 13. A method of producing oxide-silicate layers on a metal surface in a liquid electrolyte, said method comprising: immersing the metal surface in a first electrolyte, connecting a bipolar power source to the metal surface, applying power, alternating the polarity of the power, immersing the metal surface in a second electrolyte, then repeating the steps of applying power and alternating the polarity of the power. 